Josephson transmission line (JTL) system

ABSTRACT

One embodiment describes a Josephson transmission line (JTL) system. The system includes a plurality of JTL stages that are arranged in series. The system also includes a clock transformer comprising a primary inductor configured to propagate an AC clock signal and a secondary inductor arranged in a series loop with at least two of the plurality of JTL stages. The clock transformer can be configured to propagate a single flux quantum (SFQ) pulse to set a respective one of the plurality of JTL stages in response to a first phase of the AC clock signal and to reset the respective one of the plurality of JTL stages in response to a second phase of the AC clock signal that is opposite the first phase.

This invention was made with Government support under Contract No. W911NF-14-C-0115. The Government has certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to computer systems, and specifically to a Josephson transmission line (JTL) system.

BACKGROUND

Superconducting digital technology has provided computing and/or communications resources that benefit from unprecedented high speed, low power dissipation, and low operating temperature. Superconducting digital technology has been developed as an alternative to CMOS technology, and typically comprises superconductor based single flux superconducting circuitry, utilizing superconducting Josephson junctions, and can exhibit typical signal power dissipation of less than 1 nW (nanowatt) per active device at a typical data rate of 20 Gb/s (gigabytes/second) or greater, and can operate at temperatures of around 4 Kelvin. Multiple Josephson junctions and inductors can be provided in a specific arrangement to provide a Josephson transmission line to propagate data signals in superconductor computing systems.

SUMMARY

One embodiment describes a Josephson transmission line (JTL) system. The system includes a plurality of JTL stages that are arranged in series. The system also includes a clock transformer comprising a primary inductor configured to propagate an AC clock signal and a secondary inductor arranged in a series loop with at least two of the plurality of JTL stages. The clock transformer can be configured to propagate a single flux quantum (SFQ) pulse to set a respective one of the plurality of JTL stages in response to a first phase of the AC clock signal and to reset the respective one of the plurality of JTL stages in response to a second phase of the AC clock signal that is opposite the first phase.

Another embodiment includes a method for propagating an SFQ pulse in a JTL system. The method includes providing a DC bias current through a primary inductor of a bias transformer to induce a bias signal via a secondary inductor. The method also includes providing an AC clock signal through a primary inductor of each of at least one clock transformer, each of the at least one clock transformer comprising a secondary inductor arranged in series with the secondary inductor associated with the bias transformer in a respective at least one series loop with at least two of a plurality of JTL stages of the JTL system. The method further includes providing unipolar SFQ pulses at an input of the JTL system, each of the unipolar SFQ pulses being propagated through the plurality of JTL stages based on the bias signal and the AC clock signal to set one of the JTL stages in the at least one series loop and to concurrently reset another one of the JTL stages in the at least one series loop in a sequence.

Another embodiment includes a JTL system comprising a first JTL stage, a second JTL stage, a third JTL stage, and a fourth JTL stage. The first JTL stage is controlled via at least one first clock transformer comprising a respective at least one primary inductor that carries an in-phase component of an AC clock signal and a respective at least one secondary inductor that is arranged in a first series loop with the first JTL stage. The first JTL stage can be set in response to an SFQ pulse propagating through the first JTL stage at a first phase of the in-phase component and can be reset in response to the SFQ pulse propagating through the third JTL stage at a second phase of the in-phase component opposite the first phase. The second JTL stage is arranged in series with the first JTL stage. The second JTL stage can be controlled via at least one second clock transformer comprising a respective at least one primary inductor that carries a quadrature-phase component of the AC clock signal and a respective at least one secondary inductor that is arranged in a second series loop with the second JTL stage. The second JTL stage can be set in response to the SFQ pulse propagating through the second JTL stage at a first phase of the quadrature-phase component and can be reset in response to the SFQ pulse propagating through the fourth JTL stage at a second phase of the quadrature-phase component opposite the first phase. The third JTL stage is arranged in series with the second JTL stage. The third JTL stage can be controlled via the first clock transformer and is arranged in the first series loop, the third JTL stage being set in response to the SFQ pulse propagating through the third JTL stage at the second phase of the in-phase component and can be reset in response to the SFQ pulse propagating through the first JTL stage at the first phase of the in-phase component. The fourth JTL stage is arranged in series with the third JTL stage. The fourth JTL stage can be controlled via the second clock transformer and is arranged in the second series loop. The fourth JTL stage can be set in response to the SFQ pulse propagating through the fourth JTL stage at the second phase of the quadrature-phase component and can be reset in response to the SFQ pulse propagating through the second JTL stage at the first phase of the quadrature-phase component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a JTL system.

FIG. 2 illustrates an example of a JTL circuit.

FIG. 3 illustrates an example of a timing diagram.

FIG. 4 illustrates an example diagram of flux in a JTL circuit.

FIG. 5 illustrates another example diagram of flux in a JTL circuit.

FIG. 6 illustrates yet another example diagram of flux in a JTL circuit.

FIG. 7 illustrates yet a further example diagram of flux in a JTL circuit.

FIG. 8 illustrates an example of a flux-shuttle system.

FIG. 9 illustrates an example of a method for propagating unipolar SFQ pulses in a JTL system.

DETAILED DESCRIPTION

The present invention relates generally to computer systems, and specifically to a Josephson transmission line (JTL) system. The JTL system can include a plurality of JTL stages that can each be configured as superconducting quantum interference devices (SQUIDs), such that each stage can include a pair of Josephson junctions and an inductor. The JTL system can also include at least one clock transformer, one associated with each of the JTL stages, and at least one bias transformer. The bias transformer can include a primary inductor that carries a DC bias current, and thus induces a bias signal on the secondary inductor. The clock transformers can propagate an AC clock signal on primary inductors, and can thus induce the AC clock signal on the secondary inductors. The secondary inductors of the clock transformers and the secondary inductor of the bias transformer can be arranged in a series loop with the respective JTL stages. Thus, on a first phase of the AC clock signal, the series loop comprising a given one of the JTL stages can switch to one of a first flux state and a second flux state in response to a single flux quantum (SFQ) pulse propagating from the respective JTL stage to a next JTL stage of the JTL. On a second phase of the AC clock signal that is opposite the first phase, the series loop comprising the JTL stage can switch to the other of the first and second flux states based on the propagation of the SFQ pulse through another JTL stage that is coupled in a series loop with the respective JTL stage, and thus can be reset absent a negative SFQ pulse. Therefore, the JTL system can provide self-resetting of the JTL stages without providing negative SFQ pulses, and can thus propagate a unipolar SFQ pulse stream.

As an example, the AC clock signal can be configured as a quadrature signal that includes an in-phase component and a quadrature-phase component (e.g., 90° out-of-phase with respect to each other). The JTL stages can be arranged as a first JTL stage, a second JTL stage, a third JTL stage, and a fourth JTL stage that are arranged in a sequence. The first and third JTL stages can be arranged in a first series loop with secondary inductors of respective first and third clock transformers associated with the in-phase component. Similarly, the second and fourth JTL stages can be arranged in a second series loop with secondary inductors of respective second and fourth clock transformers associated with the quadrature-phase component. Therefore, the respective series loops comprising first JTL stage and the second JTL stage are set from the first flux state to the second flux state via the first clock transformer and the second clock transformer, respectively, in response to the first phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal, and the propagation of an SFQ pulse through the respective JTL stages, and are reset from the second flux state to the first flux state in response to the second phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal. Similarly, the respective series loops comprising the third JTL stage and the fourth JTL stage are set from the second flux state to the first flux state via the third clock transformer and the fourth clock transformer, respectively, in response to the second phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal, and the propagation of an SFQ pulse through the respective JTL stages, and are reset from the first flux state to the second flux state in response to the first phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal.

A first flux state, as referred to herein, describes that a loop comprising a bias transformer secondary inductor and two JTL stages (e.g. first and third or second and fourth, as described herein), encloses a magnetic flux of +Φ₀/2, and therefore a persistent current in the loop is flowing away from the Josephson junctions of one of the JTL stages in the loop and towards the Josephson junctions of the other JTL stage in the loop. A second flux state, as referred to herein, describes that the same loop encloses a magnetic flux of −Φ₀/2, and therefore the persistent current in the loop is flowing in the opposite direction relative to the first flux state, and thus towards the Josephson junctions of the other JTL stage and away from the Josephson junctions of the one JTL stage (relative to the first flux state). A JTL stage is described herein as “reset” when the persistent DC current in the respective bias transformer secondary inductor is flowing towards the Josephson junctions of the respective JTL stage. A JTL stage is described herein as “set” when the persistent current in the respective bias transformer secondary inductor is flowing away from the Josephson junctions of the respective JTL stage. Therefore, in a given flux state, one JTL stage in a given loop is in the set state and the other JTL stage is in the reset state. Accordingly, an SFQ propagating through the one JTL stage of the loop sets the respective one JTL stage, causing a transition to the second flux state and causing a reset of the other JTL stage of the loop.

FIG. 1 illustrates an example of a JTL system 10. The JTL system 10 can be implemented in any of a variety of quantum and/or classical circuit systems to propagate SFQ pulses, such as between circuit devices. As described herein, the JTL system 10 can be implemented to propagate unipolar encoded SFQ pulses, and thus does not require negative SFQ pulses as typically required in a reciprocal quantum logic (RQL) computing system. As also described herein, the term “propagate” with respect to an SFQ pulse (e.g., unipolar encoded SFQ pulse) describes an SFQ pulse being generated via the triggering of a Josephson junction in a given stage of the JTL system 10, such that the voltage of the SFQ pulse, combined with a bias voltage (e.g., via the clock signal CLK), causes a Josephson junction of the next stage in the JTL system 10 to generate another SFQ pulse, and so on. As an example, the JTL system 10 can be implemented in a rapid single flux quantum (RSFQ) logic system, or in a combination RSFQ and RQL system.

The JTL system 10 includes a plurality of JTL stages 12 that can each be associated with a given phase of an AC clock signal CLK. As an example, the AC clock signal CLK can be a quadrature clock signal, such that the AC clock signal includes an in-phase component and a quadrature-phase component. Each of the JTL stages 12 can be configured, for example, as SQUIDs. Thus, each of the JTL stages 12 can include a pair of Josephson junctions and an inductor. The JTL system 10 can also include at least one clock transformer 14, such that each of the clock transformers 14 can be associated with a respective one of the JTL stages 12. As an example, the clock transformers 14 can propagate the AC clock signal on primary inductors, and can thus induce the AC clock signal on the secondary inductors of the clock transformers 14. The secondary inductors of the clock transformers 14 can be arranged in a series loop with the respective JTL stages 12. Additionally, a DC flux bias signal BIAS can be provided to the JTL system 10 to provide a flux bias to the JTL stages 12 to facilitate the switching of flux states of the series loop, as described herein.

In the example of FIG. 1, the JTL system 10 receives SFQ pulses, demonstrated as a signal PLS_(IN), at an input. The signal PLS_(IN) can be provided as a unipolar encoded sequence of SFQ pulses, which, as described in greater detail herein, can be absent negative SFQ pulses (as demonstrated at 16). Based on the arrangement of the clock transformers 14 with respect to the JTL stages 12, on a first phase of the AC clock signal, the series loop comprising a pair of the JTL stages 12 can switch to one of a first flux state and a second flux state in response to an SFQ pulse of the signal PLS_(IN) to propagate the SFQ pulse from the respective JTL stage 12 to a next JTL stage 12 of the JTL system 10. On a second phase of the AC clock signal that is opposite the first phase (e.g., 180° out-of-phase), the series loop comprising the pair of JTL stages 12 can switch to the other of the first and second flux states based on the SFQ pulse of the signal PLS_(IN) propagating through a respective one of the JTL stages 12, and thus can be reset absent a negative SFQ pulse. Therefore, the JTL system 10 can provide self-resetting of the JTL stages 12 without providing negative SFQ pulses, and can thus propagate a unipolar SFQ pulse stream.

FIG. 2 illustrates an example of a JTL circuit 50. The JTL circuit 50 can correspond to the JTL system 10 in the example of FIG. 1, and can thus be configured to propagate a sequence of unipolar SFQ pulses. The JTL circuit 50 includes a first JTL stage 52, a second JTL stage 54, a third JTL stage 56, and a fourth JTL stage 58. The JTL stages 52, 54, 56, and 58 are sequentially arranged in series with respect to each other to propagate a signal PLS_(IN) that can be provided as a sequence of unipolar SFQ pulses. In the example of FIG. 2, the AC clock signal is demonstrated as a quadrature signal including an in-phase component CLK_(I) and a quadrature-phase component CLK_(Q). Therefore, the in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) can collectively correspond to the AC clock signal, such as can be implemented for RQL circuits.

Each of the JTL stages 52, 54, 56, and 58 are configured substantially similarly with respect to each other, and are each arranged as SQUIDs. The first JTL stage 52 includes a first Josephson junction J₁ _(_) ₁, a second Josephson junction J₂ _(_) ₁, an inductor L_(X) _(_) ₁, and an inductor L_(Y) _(_) ₁. The second JTL stage 54 includes a first Josephson junction J₁ _(_) ₂, a second Josephson junction J₂ _(_) ₂, an inductor L_(X) _(_) ₂, and an inductor L_(Y) _(_) ₂. The third JTL stage 56 includes a first Josephson junction J₁ _(_) ₃, a second Josephson junction J₂ _(_) ₃, an inductor L_(X) _(_) ₃, and an inductor L_(Y) _(_) ₃. The fourth JTL stage 58 includes a first Josephson junction J₁ _(_) ₄, a second Josephson junction J₂ _(_) ₄, an inductor L_(X) _(_) ₄, and an inductor L_(Y) _(_) ₄. The first JTL stage 52 receives the input signal PLS_(IN) via an inductor L_(I) _(_) ₁ and is separated from the second JTL stage 54 by an inductor L_(I) _(_) ₂. The second JTL stage 54 and the third JTL stage 56 are separated by an inductor L_(I) _(_) ₃, and the third JTL stage 56 and the fourth JTL stage 58 are separated by an inductor L_(I) _(_) ₄. After the fourth JTL stage 58, the JTL circuit 50 can output SFQ pulses as an output signal PLS_(OUT).

The JTL circuit 50 also includes a plurality of clock transformers that are associated with each of the JTL stages 52, 54, 56, and 58. In the example of FIG. 2, the clock transformers include a first clock transformer T₁ that is associated with the first JTL stage 52, a second clock transformer T₂ that is associated with the second JTL stage 54, a third clock transformer T₃ that is associated with the third JTL stage 56, and a fourth clock transformer T₄ that is associated with the fourth JTL stage 58. Additionally, the JTL circuit 50 includes a first bias transformer T_(B1) that is associated with the first and third JTL stages 52 and 56, and a second bias transformer T_(B2) that is associated with the second and fourth JTL stages 54 and 58.

The clock transformers T₁ and T₃ include primary inductors L₁ _(_) ₁ and L₁ _(_) ₃, respectively, through which the in-phase component CLK_(I) flows, and the clock transformers T₂ and T₄ include primary inductors L₁ _(_) ₂ and L₁ _(_) ₄, respectively, through which the quadrature-phase component CLK_(Q) flows. In addition, the bias transformers T_(B1) and T_(B2) include primary inductors L_(B) _(_) ₁ and L_(B) _(_) ₃ through which a DC bias signal BIAS flows. The clock transformer T₁ provides inductive coupling of the in-phase component CLK_(I) to the first JTL stage 52 via a secondary inductor L₂ _(_) ₂ that is coupled between the inductors L_(X) _(_) ₁ and L_(Y) _(_) ₁. Similarly, the clock transformer T₃ provides inductive coupling of the in-phase component CLK_(I) to the third JTL stage 56 via a secondary inductor L₂ _(_) ₃ that is coupled between the inductors L_(X) _(_) ₃ and L_(Y) _(_) ₃. In the example of FIG. 2, the bias transformer T_(B1) includes a secondary inductor L_(B) _(_) ₂ that is arranged in series with the secondary inductors L₂ _(_) ₁ and L₂ _(_) ₃ to form a series loop between the first and third JTL stages 52 and 56. In a similar manner, the clock transformer T₂ provides inductive coupling of the quadrature-phase component CLK_(Q) to the second JTL stage 54 via a secondary inductor L₂ _(_) ₂ that is coupled between the inductors L_(X) _(_) ₂ and L_(Y) _(_) ₂. Similarly, the clock transformer T₄ provides inductive coupling of the quadrature-phase component CLK_(Q) to the fourth JTL stage 58 via a secondary inductor L₂ _(_) ₄ that is coupled between the inductors L_(X) _(_) ₄ and L_(Y) _(_) ₄. In the example of FIG. 2, the bias transformer T_(B2) includes a secondary inductor L_(B) _(_) ₄ that is arranged in series with the secondary inductors L₂ _(_) ₂ and L₂ _(_) ₄ to form a series loop between the second and fourth JTL stages 54 and 58.

Based on the arrangement of the clock transformers T₁, T₂, T₃, and T₄ relative to the respective JTL stages 52, 54, 56, and 58, a flux state of the series loops comprising the JTL stages 52, 54, 56, and 58 can be sequentially switched at each phase and each opposite phase of the in-phase and quadrature-phase components CLK_(I) and CLK_(Q). As an example, each of the first in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) can include a first phase corresponding to a positive peak (e.g., in a first half of a respective period) and a second phase that is opposite the first phase, and thus corresponding to a negative peak (e.g., in a second half of a respective period). Therefore, as described in greater detail in the examples of FIGS. 3-7, the secondary inductors L₂ _(_) ₁, L₂ _(_) ₂, L₂ _(_) ₃, and L₂ _(_) ₄ of the clock transformers T₁, T₂, T₃, and T₄ can sequentially provide current in each of 90° intervals of the respective in-phase and quadrature-phase components CLK_(I) and CLK_(Q) to sequentially switch the flux states of the series loops comprising the JTL stages 52, 54, 56, and 58 in response to propagation of the SFQ pulse through each of the respective JTL stages 52, 54, 56, and 58. Thus, the JTL circuit 50 can track the flux state of the secondary inductors L_(B) _(_) ₂ and L_(B) _(_) ₄ of the bias transformers T_(B1) and T_(B2), and thus the flux state of the series loops comprising the JTL stages 52, 54, 56, and 58 through each alternating first and second phase of the in-phase and quadrature-phase components CLK_(I) and CLK_(Q) to propagate an SFQ pulse through the JTL circuit 50 in one of the JTL stages 52, 54, 56, and 58 to set the flux state of the respective pair of the JTL stages 52, 54, 56, and 58 and to concurrently reset the flux state of the other of the JTL stages 52, 54, 56, and 58 in the same series loop.

FIGS. 3-7 illustrate example diagrams of flux in the JTL circuit 50 of the example of FIG. 2 at each of different phases of the AC clock signal CLK. As described in greater detail herein, FIG. 3 illustrates an example diagram 100 of an initial flux state of the JTL circuit 50. FIG. 4 illustrates an example diagram 102 of the flux of the JTL circuit 50 at the first phase of the in-phase component CLK_(I), and FIG. 5 illustrates an example diagram 104 of the flux of the JTL circuit 50 at the first phase of the quadrature-phase component CLK_(Q). Similarly, FIG. 6 illustrates an example diagram 106 of the flux of the JTL circuit 50 at the second phase of the in-phase component CLK_(I), and FIG. 7 illustrates an example diagram 108 of the flux of the JTL circuit 50 at the second phase of the quadrature-phase component CLK_(Q).

The diagrams 100, 102, 104, 106, and 108 include the waveform of the AC clock signal CLK, demonstrated as the in-phase component CLK_(I) and the quadrature-phase component CLK_(Q), as indicated at a legend 101, as a function of time. The in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) are each demonstrated as sinusoidal signals having magnitudes centered about zero. The in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) in the example of FIGS. 3-7 can correspond to the in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) in the example of FIG. 2. The JTL circuit 50 is demonstrated simplistically in the examples of FIGS. 3-7, and thus without the clock transformers T₁, T₂, T₃, and T₄, without the bias transformers T_(B1) and T_(B2), and without any of the inductors in and between the JTL stages 52, 54, 56, and 58. Therefore, the diagrams merely demonstrate a first series loop 110 associated with the first and third JTL stages 52 and 56, and a second series loop 112 associated with the second and fourth JTL stages 54 and 58. In the following description, reference is to be made to the examples of FIGS. 2-7.

As an example, the signal PLS_(IN) can provide an SFQ pulse to the inductor L_(I) _(_) ₁ at approximately the time t₀. At approximately the time t₀, the first series loop 110, formed by the first JTL stage 52, the secondary inductors L₂ _(_) ₁, L_(B) _(_) ₂, L₂ _(_) ₃, and the third JTL stage 56, has a first flux state (e.g., encloses a flux) of +Φ₀/2, as indicated in the diagram 100 in the example of FIG. 3. The flux state +Φ₀/2 is based on the DC flux bias induced in the secondary inductor L_(B) _(_) ₂ via the DC flux transformer T_(B1). At the time t₀, a positive portion of the in-phase component CLK_(I) begins, with the first phase (e.g., positive peak) of the in-phase component CLK_(I) occurring at a time t₁ (FIG. 4). Therefore, the in-phase component CLK_(I) begins to induce a clock current via the secondary inductors L₂ _(_) ₁ and L₂ _(_) ₃ in a first direction, with current flowing towards the Josephson junctions J₁ _(_) ₁ and J₂ _(_) ₁ of the first JTL stage 52 based on the inductive coupling with the respective primary inductors L₁ _(_) ₁ and L₁ _(_) ₃. At a time just prior to the time t₁, the induced clock current is combined with the induced bias signal (e.g., via the DC bias current BIAS provided via the secondary inductor L_(B) _(_) ₂) and the SFQ pulse indicated at 114 in the diagram 102 in the example of FIG. 4. Therefore, the critical current of the Josephson junctions J_(I) _(_) ₁ and J₂ _(_) ₁ is exceeded by the combined current to trigger the Josephson junctions J₁ _(_) ₁ and J₂ _(_) ₁, thus propagating the SFQ pulse to the second JTL stage 54 via the inductor L_(I) _(_) ₂ and switching the flux state of the first series loop 110 from the first flux state of +Φ₀/2 to the second flux state of −Φ₀/2. Additionally, as described in greater detail herein, the switching of the flux state of the first series loop 110 from the first flux state of +Φ₀/2 to the second flux state of −Φ₀/2 resets the third JTL stage 56.

At approximately the time t₁, the second series loop 112, formed by the second JTL stage 54, the secondary inductors L₂ _(_) ₂, L_(B) _(_) ₄, L₂ _(_) ₄, and the fourth JTL stage 58, has the first flux state (e.g., encloses a flux) of +Φ₀/2, based on the DC flux bias induced in the secondary inductor L_(B) _(_) ₄ via the DC flux transformer T_(B2), as indicated in the diagram 102 in the example of FIG. 4. At the time t₁, a positive portion of the quadrature-phase component CLK_(Q) begins, with the first phase (e.g., positive peak) of the quadrature-phase component CLK_(Q) occurring at a time t₂ (FIG. 5). Therefore, the quadrature-phase component CLK_(Q) begins to induce a clock current via the secondary inductors L₂ _(_) ₂ and L₂ _(_) ₄ in a first direction with current flowing towards the Josephson junctions J₁ _(_) ₂ and J₂ _(_) ₂ of the second JTL stage 54 based on the inductive coupling with the respective primary inductors L₁ _(_) ₂ and L₁ _(_) ₄. At a time just prior to the time t₂, the induced clock current is combined with the induced bias signal (e.g., via the DC bias current BIAS provided via the secondary inductor L_(B) _(_) ₄) and the SFQ pulse. Therefore, the critical current of the Josephson junctions J₁ _(_) ₂ and J₂ _(_) ₂ is exceeded by the combined current to trigger the Josephson junctions J₁ _(_) ₂ and J₂ _(_) ₂, thus propagating the SFQ pulse indicated at 116 in the diagram 104 in the example of FIG. 5 to the third JTL stage 56 via the inductor L_(I) _(_) ₃ and switching the flux state of the second series loop 112 from the first flux state of +Φ₀/2 to the second flux state of −Φ₀/2. Additionally, as described in greater detail herein, the switching of the flux state of the second series loop 112 from the first flux state of +Φ₀/2 to the second flux state of −Φ₀/2 resets the fourth JTL stage 58.

At approximately the time t₂, the first series loop 110 has the second flux state of −Φ₀/2, as indicated in the diagram 104 in the example of FIG. 5, based on the DC flux bias induced in the secondary inductor L_(B) _(_) ₂ via the DC flux transformer T_(B1) and based on the SFQ having propagated through the first JTL stage 52. At the time t₂, a negative portion of the in-phase component CLK_(I) begins, with the second phase (e.g., negative peak) of the in-phase component CLK_(I) occurring at a time t₃ (FIG. 6). Therefore, the in-phase component CLK_(I) begins to induce a clock current via the secondary inductors L₂ _(_) ₁ and L₂ _(_) ₃ in a second direction, with current flowing towards the Josephson junctions J₁ _(_) ₃ and J₂ _(_) ₃ of the third JTL stage 56 based on the inductive coupling with the respective primary inductors L₁ _(_) ₁ and L₁ _(_) ₃. At a time just prior to the time t₃, the induced clock current is combined with the induced bias signal (e.g., via the DC bias current BIAS provided via the secondary inductor L_(B) _(_) ₂) and the SFQ pulse. Therefore, the critical current of the Josephson junctions J₁ _(_) ₃ and J₂ _(_) ₃ is exceeded by the combined current to trigger the Josephson junctions J₁ _(_) ₃ and J₂ _(_) ₃, thus propagating the SFQ pulse indicated at 118 in the diagram 106 in the example of FIG. 6 to the fourth JTL stage 58 via the inductor L_(I) _(_) ₄ and switching the flux state of the first series loop 110 from the second flux state of −Φ₀/2 to the first flux state of +Φ₀/2.

In addition, because the flux state of the first series loop 110 is switched from the second flux state of −Φ₀/2 to the first flux state of +Φ₀/2, the first JTL stage 52 is reset absent a negative SFQ pulse. In other words, unlike in typical RQL circuit systems that require a negative SFQ pulse to discharge inductive energy and reset the bistable flux state of the inductive bias loop, the JTL circuit 50 is self-resetting with respect to the JTL stages 52, 54, 56, and 58. Therefore, the JTL circuit 50 can propagate the input signal PLS_(IN) as a sequence of unipolar SFQ pulses, and thus does not require negative SFQ pulses to reset the respective JTL stages 52, 54, 56, and 58 upon propagating a SFQ pulse along the respective JTL formed by the JTL stages 52, 54, 56, and 58. The third JTL stage 56 was similarly reset at the time t₁, as demonstrated in the diagram 102 of the example of FIG. 4, in which the third JTL stage 56 is reset at approximately the time t₁, and thus substantially concurrently with the setting of the first JTL stage 52.

At approximately the time t₃, the second series loop 112 has the second flux state of −Φ₀/2, as indicated in the diagram 106 in the example of FIG. 6. At the time t₃, a negative portion of the quadrature-phase component CLK_(Q) begins, with the second phase of the quadrature-phase component CLK_(Q) occurring at a time t₄ (FIG. 7). Therefore, the quadrature-phase component CLK_(Q) begins to induce a clock current via the secondary inductors L₂ _(_) ₂ and L₂ _(_) ₄ in the second direction, with current flowing towards the Josephson junctions J₁ _(_) ₄ and J₂ _(_) ₄ of the fourth JTL stage 58 based on the inductive coupling with the respective primary inductors L₁ _(_) ₂ and L₁ _(_) ₄. At a time just prior to the time t₄, the induced clock current is combined with the induced bias signal (e.g., via the DC bias current BIAS provided via the secondary inductor L_(B) _(_) ₄) and the SFQ pulse. Therefore, the critical current of the Josephson junctions J₁ _(_) ₄ and J₂ _(_) ₄ is exceeded by the combined current to trigger the Josephson junctions J₁ _(_) ₄ and J₂ _(_) ₄, thus propagating the SFQ pulse, as indicated at 126 in the diagram 108 in the example of FIG. 7 as the output signal PLS_(OUT) and switching the flux state of the second series loop 112 from the second flux state of −Φ₀/2 to the first flux state of +Φ₀/2.

In addition, because the flux state of the second series loop 112 is switched from the second flux state of −Φ₀/2 to the first flux state of +Φ₀/2, the second JTL stage 54 is reset absent a negative SFQ pulse. The fourth JTL stage 58 was similarly reset at the time t₂, as demonstrated in the diagram 104 of the example of FIG. 5, and thus substantially concurrently with the setting of the second JTL stage 54.

Therefore, the examples of FIGS. 2-7 demonstrate a JTL circuit 50 that can propagate the input signal PLS_(IN) as a sequence of unipolar SFQ pulses, and can thus provide self-resetting of the JTL stages 52, 54, 56, and 58 without implementing negative SFQ pulses based on manipulating the flux states of the series loops 158 and 160 that include the respective JTL stages 52, 54, 56, and 58. It is to be understood that the JTL circuit 50 is not intended to be limited to the examples of FIGS. 2-7. As an example, the series order of the transformers T₁, T_(B1), and T₃ with respect to the series loop 158, and the transformers T₂, T_(B2), and T₄ with respect to the series loop 160, is not limited to as demonstrated in the example of FIG. 2, but could be any equivalent series order. Additionally, it is to be understood that, while the transformers T₁ and T₃ and the transformers T₂ and T₄ are demonstrated as separate transformers having approximately equal mutual inductance, it is to be understood that an equivalent single transformer could be implemented instead of the separate transformers T₁ and T₃ and/or the separate transformers T₂ and T₄, with the single transformer having approximately twice the mutual inductance as the separate transformers T₁ and T₃ and/or the separate transformers T₂ and T₄. Additionally, the JTL circuit 50 can be a single JTL segment, such that multiple JTL segments can be combined in series to provide a JTL that extends through multiple periods of the AC clock signal CLK. Accordingly, the JTL circuit 50 can be configured in a variety of ways.

FIG. 8 illustrates an example of a flux-shuttle system 200. The flux-shuttle system 200 can correspond to a Josephson AC/DC converter that is configured to convert an AC input signal (e.g., the AC clock signal CLK) to a DC output signal. The flux-shuttle system 200 includes a first loop stage 202, a second loop stage 204, a third loop stage 206, and a fourth loop stage 208. The loop stages 202, 204, 206, and 208 are configured substantially similar to the JTL stages 52, 54, 56, and 58 in the example of FIG. 2, and thus are demonstrated in the example of FIG. 8 as SQUIDs. The loop stages 202, 204, 206, and 208 are sequentially arranged in series with respect to each other, and are arranged in a loop, such that the loop stage 208 is coupled in series to the loop stage 202 via an initialization system 210. The initialization system 210 can be any of a variety of arrangements to inject an SFQ pulse into the flux-shuttle system 200 to propagate sequentially through the loop stages 202, 204, 206, and 208, as described in greater detail herein. In the example of FIG. 8, the AC clock signal is demonstrated as a quadrature signal including an in-phase component CLK_(I) and a quadrature-phase component CLK_(Q). Therefore, the in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) can collectively correspond to the AC clock signal, such as can be implemented for RQL circuits.

Each of the loop stages 202, 204, 206, and 208 are configured substantially similarly with respect to each other, and are each arranged as SQUIDs. The first loop stage 202 includes a first Josephson junction J₁ _(_) ₁, a second Josephson junction J₂ _(_) ₁, an inductor L_(X) _(_) ₁, and an inductor L_(Y) _(_) ₁. The second loop stage 204 includes a first Josephson junction J₁ _(_) ₂, a second Josephson junction J₂ _(_) ₂, an inductor L_(X) _(_) ₂, and an inductor L_(Y) _(_) ₂. The third loop stage 206 includes a first Josephson junction J₁ _(_) ₃, a second Josephson junction J₂ _(_) ₃, an inductor L_(X) _(_) ₃, and an inductor L_(Y) _(_) ₃. The fourth loop stage 208 includes a first Josephson junction J₁ _(_) ₄, a second Josephson junction J₂ _(_) ₄, an inductor L_(X) _(_) ₄, and an inductor L_(Y) _(_) ₄. The first loop stage 202 receives an SFQ pulse from either the initialization system 210 or from the fourth loop stage 208 via an inductor L_(I) _(_) ₁ and is separated from the second loop stage 204 by an inductor L_(I) _(_) ₂. The second loop stage 204 and the third loop stage 206 are separated by an inductor L_(I) _(_) ₃, and the third loop stage 206 and the fourth loop stage 208 are separated by an inductor L_(I) _(_) ₄. After the fourth loop stage 208, the flux-shuttle system 200 can output the SFQ pulse back to the first loop stage 202. Therefore, the SFQ pulse that is generated in the initialization system 210 can circulate through the flux-shuttle system 200 in the loop formed by the loop stages 202, 204, 206, and 208.

The flux-shuttle system 200 also includes a plurality of clock transformers that are associated with each of the loop stages 202, 204, 206, and 208. In the example of FIG. 8, the clock transformers include a first clock transformer T₁ that is associated with the first loop stage 202, a second clock transformer T₂ that is associated with the second loop stage 204, a third clock transformer T₃ that is associated with the third loop stage 206, and a fourth clock transformer T₄ that is associated with the fourth loop stage 208. Additionally, the flux-shuttle system 200 includes a first bias transformer T_(B1) and a second bias transformer T_(B2) that are associated with the first and third loop stages 202 and 206, and includes a third bias transformer T_(B3) and a fourth bias transformer T_(B4) that is associated with the second and fourth loop stages 204 and 208.

The clock transformers T₁ and T₃ include primary inductors L₁ _(_) ₁ and L₁ _(_) ₃, respectively, through which the in-phase component CLK_(I) flows, and the clock transformers T₂ and T₄ include primary inductors L₁ _(_) ₂ and L₁ _(_) ₄, respectively, through which the quadrature-phase component CLK_(Q) flows. In addition, the bias transformers T_(B1), T_(B2), T_(B3), and T_(B4) include primary inductors L_(B) _(_) ₁, L_(B) _(_) ₃, L_(B) _(_) ₅, and L_(B) _(_) ₇ through which a DC bias signal BIAS flows. The clock transformer T₁ provides inductive coupling of the in-phase component CLK_(I) to the first loop stage 202 via a secondary inductor L₂ _(_) ₁ that is coupled between the inductors L_(X) _(_) ₁ and L_(Y) _(_) ₁. Similarly, the clock transformer T₃ provides inductive coupling of the in-phase component CLK_(I) to the third loop stage 206 via a secondary inductor L₂ _(_) ₃ that is coupled between the inductors L_(X) _(_) ₃ and L_(Y) _(_) ₃. In the example of FIG. 8, the bias transformers T_(B1) and T_(B2) include respective secondary inductors L_(B) _(_) ₂ and L_(B) _(_) ₄ that are arranged in series with each other and with the secondary inductors L₂ _(_) ₁ and L₂ _(_) ₃ to form a first series loop 212 between the first and third loop stages 202 and 206. In a similar manner, the clock transformer T₂ provides inductive coupling of the quadrature-phase component CLK_(Q) to the second loop stage 204 via a secondary inductor L₂ _(_) ₂ that is coupled between the inductors L_(X) _(_) ₂ and L_(Y) _(_) ₂. Similarly, the clock transformer T₄ provides inductive coupling of the quadrature-phase component CLK_(Q) to the fourth loop stage 208 via a secondary inductor L₂ _(_) ₄ that is coupled between the inductors L_(X) _(_) ₄ and L_(Y) _(_) ₄. In the example of FIG. 8, the bias transformers T_(B3) and T_(B4) include respective secondary inductors L_(B) _(_) ₆ and L_(B) _(_) ₈ that are arranged in series with the secondary inductors L₂ _(_) ₂ and L₂ _(_) ₄ to form a second series loop 214 between the second and fourth loop stages 204 and 208.

Similar to as described previously regarding the example of FIGS. 2-7, based on the arrangement of the clock transformers T₁, T₂, T₃, and T₄ relative to the respective loop stages 202, 204, 206, and 208, a flux state of the series loops 212 and 214 comprising the respective loop stages 202, 204, 206, and 208 can be sequentially switched at each phase and each opposite phase of the in-phase and quadrature-phase components CLK_(I) and CLK_(Q). As an example, each of the first in-phase component CLK_(I) and the quadrature-phase component CLK_(Q) can include a first phase corresponding to a positive peak (e.g., in a first half of a respective period) and a second phase that is opposite the first phase, and thus corresponding to a negative peak (e.g., in a second half of a respective period). Thus, the flux-shuttle system 200 can track the flux state of the secondary inductors L_(B) _(_) ₂ and L_(B) _(_) ₄ of the bias transformers T_(B1) and T_(B2) in the first series loop 212 and the flux state of the secondary inductors L_(B) _(_) ₆ and L_(B) _(_) ₈ of the bias transformers T_(B3) and T_(B4) in the second series loop 214, and thus the flux state of the series loops 212 and 214 comprising the respective loop stages 202, 204, 206, and 208 through each alternating first and second phase of the in-phase and quadrature-phase components CLK_(I) and CLK_(Q). Accordingly, the flux shuttle system 200 can propagate an SFQ pulse in one of the JTL stages 202, 204, 206, and 208 to set one of the JTL stages 202, 204, 206, and 208 and to concurrently reset one of the JTL stages 202, 204, 206, and 208 in the same one of the series loops 212 and 214.

Furthermore, the flux-shuttle system 200 includes a first storage inductor L_(S) _(_) ₁ that interconnects an output node 216 and the first series loop 212, and a second storage inductor L_(S) _(_) ₂ that interconnects the output node 216 and the second series loop 214. The flux-shuttle system 200 further includes an output inductor L_(OUT) that conducts an output current I_(OUT) from the output node 216. In response to the SFQ pulse that is sequentially propagated through each of the loop stages 202, 204, 206, and 208, a current step is generated in the respective storage inductors L_(S) _(_) ₁ and L_(S) _(_) ₂. Thus, in response to the respective Josephson junctions triggering in the first and third loop stages 202 and 206 to the switch of the flux state of the series loop 212, the SFQ pulse generates a resulting current step in the storage inductor L_(S) _(_) ₁. Similarly, in response to the respective Josephson junctions triggering in the second and fourth loop stages 204 and 208 to the switch the flux state of the series loop 214, the SFQ pulse generates a resulting current step in the storage inductor L_(S) _(_) ₂. As a result, the output inductor L_(OUT) integrates each of the current steps provided through the storage inductors L_(S) _(_) ₁ and L_(S) _(_) ₂ to provide the output current I_(OUT), such that the flux-shuttle loop 200 acts as a DC signal source. As a result, the output current I_(OUT) can be provided as a DC signal converted from the in-phase component CLK_(I) and the quadrature-phase component CLK_(Q), such as to a circuit device (e.g., a peripheral device in a memory system).

Therefore, the example of FIG. 8 demonstrates a flux-shuttle system 200 that can propagate an SFQ pulse and provide self-resetting of the loop stages 202, 204, 206, and 208, and thus without implementing a negative SFQ pulse to reset the flux state of the bias transformers T_(B1), T_(B2), T_(B3), and T_(B4) in the flux-shuttle system 200. It is to be understood that the flux-shuttle system 200 is not intended to be limited to the example of FIG. 8, such as in ways similar to as described previously regarding the examples of FIGS. 2-7. As example, the initialization system 210 is not limited to being arranged between the fourth JTL stage 208 and the first JTL stage 202, but could instead be arranged between any two of the JTL stages 202, 204, 206, and 208. Accordingly, the flux-shuttle system 200 can be configured in a variety of ways

In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to FIG. 9. While, for purposes of simplicity of explanation, the methodology of FIG. 9 is shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects could, in accordance with the present invention, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect of the present invention.

FIG. 9 illustrates an example of a method 250 for propagating unipolar SFQ pulses in a JTL system (e.g., the JTL system 10). At 252, a DC bias current (e.g., the DC bias current BIAS) is provided through a primary inductor (e.g., primary inductors L_(B) _(_) ₁ and L_(B) _(_) ₃) of a bias transformer (e.g., the bias transformers T_(B1) and T_(B2)) to induce a bias signal via a secondary inductor (e.g., secondary inductors L_(B) _(_) ₂ and L_(B) _(_) ₄). At 254, an AC clock signal (e.g., the AC clock signal CLK) is provided through a primary inductor (e.g., primary inductors L₁ _(_) ₁ and L₁ _(_) ₃ and the primary inductors L₁ _(_) ₂ and L₁ _(_) ₄) of each of at least one clock transformer (e.g., the clock transformers T₁, T₂, T₃, and T₄). The at least one clock transformer can include secondary inductors (e.g., secondary inductors L₂ _(_) ₁ and L₂ _(_) ₃ and the secondary inductors L₂ _(_) ₂ and L₂ _(_) ₄) arranged in series with the secondary inductor associated with the bias transformer in a respective at least one series loop with at least two of a plurality of JTL stages (e.g., the JTL stages 52, 54, 56, and 58) of the JTL system. At 256, providing unipolar SFQ pulses (e.g., via the input signal PLS_(IN)) at an input of the JTL system, each of the unipolar SFQ pulses being propagated through the plurality of JTL stages based on the bias signal and the AC clock signal to set one of the JTL stages in the at least one series loop and to concurrently reset another one of the JTL stages in the at least one series loop in a sequence.

What have been described above are examples. It is, of course, not possible to describe every conceivable combination of components or methods, but one of ordinary skill in the art will recognize that many further combinations and permutations are possible.

Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements. As used herein, the term “includes” means includes but not limited to, and the term “including” means including but not limited to. The term “based on” means based at least in part on. 

What is claimed is:
 1. A Josephson transmission line (JTL) system comprising: a plurality of JTL stages that are arranged in series; and a clock transformer comprising a primary inductor configured to propagate an AC clock signal and a secondary inductor arranged in a series loop with at least two of the plurality of JTL stages, the clock transformer being configured to propagate a single flux quantum (SFQ) pulse to set a respective one of the plurality of JTL stages in response to a first phase of the AC clock signal and to reset the respective one of the plurality of JTL stages in response to a second phase of the AC clock signal that is opposite the first phase.
 2. The system of claim 1, wherein the AC clock signal is a quadrature clock signal comprising an in-phase component and a quadrature-phase component, the in-phase and quadrature phase components being out-of-phase by approximately 90°.
 3. The system of claim 2, wherein the plurality of JTL stages comprises a first JTL stage coupled to at least one first clock transformer, a second JTL stage coupled to at least one second clock transformer, a third JTL stage coupled to the at least one first clock transformer, and a fourth JTL stage coupled to the at least one second clock transformer, wherein each of the at least one first clock transformer comprises a primary inductor configured to propagate the in-phase component of the AC clock signal, wherein each of the at least one second clock transformer comprises a primary inductor configured to propagate the quadrature-phase component of the AC clock signal.
 4. The system of claim 3, wherein the first JTL stage and the second JTL stage are set by the propagation of the SFQ pulse through the first JTL stage and the second JTL stage, respectively, in response to the first phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal, and are reset based on the propagation of the SFQ pulse through the third JTL stage and the fourth JTL stage, respectively, in response to the second phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal, and wherein the third JTL stage and the fourth JTL stage are set by the propagation of the SFQ pulse through the third JTL stage and the fourth JTL stage, respectively, in response to the second phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal, and are reset based on the propagation of the SFQ pulse through the first JTL stage and the second JTL stage, respectively, in response to the first phase of the in-phase component and the quadrature-phase component, respectively, of the AC clock signal.
 5. The system of claim 1, further comprising a bias transformer comprising a primary inductor configured to carry a DC bias signal and comprising a secondary inductor that is arranged in series with the secondary inductor of the clock transformer in the series loop, the DC bias signal being inductively provided to the plurality of JTL stages to set one of the plurality of JTL stages in response to the SFQ pulse propagating through the one of the plurality of JTL stages at the first phase of the AC clock signal to propagate the SFQ pulse and to reset the one of the plurality of JTL stages in response to the SFQ pulse propagating through another one of the plurality of JTL stages in the series loop at the second phase of the AC clock signal.
 6. The system of claim 5, wherein the bias transformer is a first bias transformer, and wherein the clock transformer comprises at least one first clock transformer associated with an in-phase portion of the AC clock signal and comprising secondary inductors arranged in a series loop with a first portion of the plurality of JTL stages, the system further comprising: at least one second clock transformer associated with a quadrature-phase portion of the AC clock signal and comprising secondary inductors arranged in a series loop with a second portion of the plurality of JTL stages; and a second bias transformer comprising a primary inductor configured to propagate the DC bias signal and comprising a secondary inductor that is arranged in series with the secondary inductor of the second clock transformer in the series loop with the second portion of the plurality of JTL stages.
 7. The system of claim 1, wherein the clock transformer is arranged with respect to the AC clock signal such that the plurality of transformers are configured to set one of the plurality of JTL stages in response to the SFQ pulse propagating through the one of the plurality of JTL stages at the first phase of the AC clock signal, and to reset the respective one of the plurality of JTL stages in response to the SFQ pulse propagating through another one of the plurality of JTL stages in the series loop at the second phase of the AC clock signal absent a negative SFQ pulse.
 8. The system of claim 1, wherein each of the plurality of JTL stages comprises a first Josephson junction, a second Josephson junction, and an inductor interconnecting the first and second Josephson junctions and being coupled to the secondary inductor of a respective one of the clock transformer, the first and second Josephson junctions and the inductor being arranged as a superconducting quantum interference device (SQUID).
 9. The system of claim 1, wherein the plurality of JTL stages are arranged in a series loop to form a flux-shuttle, wherein each of the plurality of JTL stages comprises at least one Josephson junction, the plurality of JTL stages being spaced about the flux shuttle loop and being configured to sequentially trigger the respective at least one Josephson junction in response to the AC clock signal and to propagate the SFQ pulse sequentially and continuously through each of the plurality of JTL stages around the flux-shuttle loop via each of the Josephson junction of each of the respective plurality of JTL stages to provide a DC output signal through an output inductor.
 10. The system of claim 9, further comprising an initialization component arranged in the series with the plurality of JTL stages in the flux-shuttle, the initialization component being configured to inject the SFQ pulse into the flux-shuttle.
 11. A method for propagating unipolar single flux quantum (SFQ) pulses in a Josephson transmission line (JTL) system, the method comprising: providing a DC bias current through a primary inductor of a bias transformer to induce a bias signal via a secondary inductor; providing an AC clock signal through a primary inductor of each of at least one clock transformer, each of the at least one clock transformer comprising a secondary inductor arranged in series with the secondary inductor associated with the bias transformer in a respective at least one series loop with at least two of a plurality of JTL stages of the JTL system; and providing unipolar SFQ pulses at an input of the JTL system, each of the unipolar SFQ pulses being propagated through the plurality of JTL stages based on the bias signal and the AC clock signal to set one of the JTL stages in the at least one series loop and to concurrently reset another one of the JTL stages in the at least one series loop in a sequence.
 12. The method of claim 11, wherein providing the AC clock signal comprises: providing an in-phase component of the AC clock signal; and providing a quadrature-phase component of the AC clock signal, the in-phase and quadrature phase components being out-of-phase by approximately 90°.
 13. The method of claim 12, wherein the plurality of JTL stages comprises a first JTL stage coupled to at least one first clock transformer, a second JTL stage coupled to at least one second clock transformer, a third JTL stage coupled to the first clock transformer, and a fourth JTL stage coupled to the second clock transformer, wherein providing the in-phase component comprises providing the in-phase component of the AC clock signal to each of the at least one first clock transformer, wherein providing the quadrature-phase component comprises providing the quadrature-phase component of the AC clock signal to each of the at least one second clock transformer.
 14. The method of claim 13, wherein, in response to a respective one of the unipolar SFQ pulses propagating through the first JTL stage, the first JTL stage is set and the third JTL stage is reset based on the first phase of the in-phase component of the AC clock signal and the bias signal; wherein, in response to the respective one of the unipolar SFQ pulses propagating through the second JTL stage, the second JTL stage is set and the fourth JTL stage is reset based on the first phase of the quadrature-phase component of the AC clock signal and the bias signal; wherein, in response to the respective one of the unipolar SFQ pulses propagating through the third JTL stage, the third JTL stage is set and the first JTL stage is reset based on the second phase of the in-phase component of the AC clock signal and the bias signal; and wherein, in response to the respective one of the unipolar SFQ pulses propagating through the fourth JTL stage, the fourth JTL stage is set and the second JTL stage is reset based on the second phase of the quadrature-phase component of the AC clock signal and the bias signal.
 15. The method of claim 11, wherein the plurality of JTL stages are arranged in the series loop to form a flux-shuttle, wherein each of the plurality of JTL stages comprises at least one Josephson junction, the plurality of JTL stages being spaced about the flux shuttle loop, wherein providing the respective one of the unipolar SFQ pulses comprises sequentially propagating a respective one of the unipolar SFQ pulses sequentially and continuously through each of the plurality of JTL stages around the flux-shuttle loop via each of the Josephson junction of each of the respective plurality of JTL stages to provide a DC output signal through an output inductor. 